Corrosion-Resistant Steel Excellent in Toughness of Base Metal and Weld Portion, and Method of Manufacturing the Same

ABSTRACT

A corrosion-resistant steel excellent in toughness of a base metal and a weld portion said steel slab contains, in % by weight, C: 0.2% or less; Si: 0.01 to 2.0%; Mn: 0.1 to 4% or less; P: 0.03% or less; S: 0.01% or less; Cr: 3 to 11%; Al: 0.1 to 2%; and N: 0.02%, and has values of 1150 or above, and 600 or above respectively for Tp and Tc expressed by the equations below using concentrations of Cr, Al, C, Mn, Cu and Ni respectively given as % Cr, % Al, % C, % Mn, % Cu and % Ni. Tp=1601−(34% Cr+287% Al)+(500% C+33% Mn+60% Cu+107% Ni); and Tc=910+80% Al−(300% C+80% Mn+15% Cr+55% Ni).

TECHNICAL FIELD

The present invention relates to a corrosion-resistant steel excellentin toughness of a base metal and a weld portion, and a method ofmanufacturing the same, and more specifically, a corrosion-resistantsteel used in various forms under various corrosive environments, suchas various containers, vacuum vessels, low-temperature heat exchangersand bathroom components used under corrosive environment with dewing orunder indoor environment; such as bridge, support columns, tunnelreinforcing components, interior and exterior materials for buildings,roof materials and fittings used under aerial corrosive environment;such as various reinforcing structures and support columns used undercorrosive environment with concrete; and such as marine vessels,bridges, piles, sheet piles and marine structures used under corrosiveenvironments with seawater.

BACKGROUND ART

Steels used under various corrosive environments, such ashigh-temperature and high-humidity corrosive environment, corrosiveenvironment with dewing, aerial corrosive environment, corrosiveenvironment with city water, corrosive environment with soil, corrosiveenvironment with concrete, and corrosive environment with seawater, aregenerally provided with some anti-corrosion measures. In recent years,in view of improving reliability, simplifying manufacturing andapplication processes, achieving maintenance-free, saving resources andthe like, there have been increasing trends in using Cr-containingsteels and stainless steels, for the purpose of improving corrosionresistance of the steel base. Most of conventional techniques have,however, failed in providing a practical measure from an economicalpoint of view, because improvement in the corrosion resistance hasresulted in increase in material cost, and have sometimes resulted inonly a limited range of applications due to poor strength whenaustenitic steels were used.

As seen in the above-described examples, any steels containing certainlevels of Cr generally became more likely to cause local corrosion asthe corrosive environment became more severe, so that as acountermeasure for this problem, further increase in the concentrationof Cr or Mo has been a most general technical means for improving theresistivity against corrosion.

In recent years, there have been proposed steels added with Al besidesCr, aiming at improving the corrosion resistance, or both of thecorrosion resistance and workability, as disclosed in Japanese PatentApplication Laid-Open Nos. 5-279791, 6-179949, 6-179950, 6-179951,6-212256, 6-212257, 7-3388 and 11-350082 and the like. These steels maybe effective to some degree in terms of improvement in the corrosionresistance or both of the corrosion resistance and the workability, butpoor in toughness of the base metal and the heat affected zone (HAZ),and this raises a tough obstacle for the steel to be applied to weldstructures.

SUMMARY OF THE INVENTION

After considering the above-described situations, the present inventionis aimed at providing a low-cost, corrosion-resistant steel showing alarge corrosion resistance under various corrosive environments such ascorrosive environment with dewing, aerial corrosive environment,corrosive environment with city water, and corrosive environment withseawater, and excellent in the toughness in the heat affected zone(HAZ).

Aiming at achieving the above-described objects, the present inventorsmade extensive studies from every aspect, in order to develop a steelshowing excellent corrosion resistance under various corrosiveenvironments such as corrosive environment with dewing, aerial corrosiveenvironment, corrosive environment with city water, corrosiveenvironment with concrete, and corrosive environment with seawater.First, after extensive investigations into techniques for improving thecorrosion resistance under the above-described various environments, aswell as the toughness of the weld portion, the present inventors foundthat a steel containing 3 to 11% of Cr, added with 0.1 to 2% of Al,showed a very excellent corrosion resistance under the above-describedvarious corrosive environments. However, this sort of steel typicallyproduces coarse ferrite when heated at 1200° C. or above during welding,due to its wide range of ferrite phase transformation, so that thetoughness may degrade to a considerable degree, and may cause cracks andthe like after welding. The present inventors then further went througha series of experiments, and found out that a mode of generation of thecoarse ferrite phase transformation during welding can be estimatedbased on a parameter Tp below, expressed using amounts of addition ofalloying elements. The parameter Tp can be expressed usingconcentrations of ferrite-forming elements (Cr, Al) andaustenite-forming elements (Mn, Ni, for example) which suppressproduction of ferrite phase. The present inventors found out thatproduction of ferrite at higher temperatures can be suppressed, when theparameter Tp has a value of not smaller than a predetermined level.

On the other hand, addition of some austenite-forming elements describedin the above can suppress production of the coarse ferrite phase in theweld portion, but addition of large amounts of the alloying elements maypromote formation of a low-temperature-transformation-forming phase withpoor toughness in the process of cooling after rolling of the basemetal, and thereby tends to lower the toughness of the base metal. Thepresent inventors then made extensive studies on preventing suchembrittlement, defined a parameter Tc which specifies concentrations ofthe alloying elements capable of ensuring a desirable level of toughnessof the base metal after rolling, and found out that a desirable level oftoughness can be ensured when the parameter Tc has a value of notsmaller than a predetermined level.

Basic concepts of the present invention are as follows.

(1) A corrosion-resistant steel excellent in toughness of a base metaland a weld portion, containing, in % by weight:

C: 0.2% or less;

Si: 0.01 to 2.0%;

Mn: 0.1 to 4%;

P: 0.03% or less;

S: 0.01% or less;

Cr: 3 to 11%;

Al: 0.1 to 2%; and

N: 0.02%, and

having values of 1150 or above, and 600 or above respectively for Tp andTc expressed by the equations below using concentrations of Cr, Al, C,Mn, Cu and Ni respectively given as % Cr, % Al, % C, % Mn, % Cu and %Ni.

Tp=1601−(34% Cr+287% Al)+(500% C+33% Mn+60% Cu+107% Ni); and

Tc=910+80% Al−(300% C+80% Mn+15% Cr+55% Ni).

(2) The corrosion-resistant steel excellent in toughness of a base metaland a weld portion according to (1), further containing, in % by weight,any one of, or two or more elements selected from the group consistingof:

Cu: 0.1 to 4%;

Ni: 0.1 to 4%;

Mo: 0.01 to 1%;

V: 0.01 to 0.1%;

Nb: 0.005 to 0.050%;

Ti: 0.005 to 0.03%;

Ca: 0.0005 to 0.05%;

Mg: 0.0005 to 0.05%; and

REM: 0.001 to 0.1%.

(3) A method of manufacturing a corrosion-resistant steel excellent intoughness of a base metal and a weld portion, including the steps of:

heating a steel slab;

the steel slab containing, in % by weight:

C: 0.2% or less;

Si: 0.01 to 2.0%;

Mn: 0.1 to 4%;

P: 0.03% or less;

S: 0.01% or less;

Cr: 3 to 11%;

Al: 0.1 to 2%; and

N: 0.02%, and

having values of 1150 or above, and 600 or above respectively for Tp andTc expressed by the equations below using concentrations of Cr, Al, C,Mn, Cu and Ni respectively given as % Cr, % Al, % C, % Mn, % Cu and %Ni,

forming a steel plate by hot rolling of the steel slab; and

cooling the steel plate by air.

Tp=1601−(34% Cr+287% Al)+(500% C+33% Mn+60% Cu+107% Ni); and

Tc=910+80% Al−(300% C+80% Mn+15% Cr+55% Ni).

(4) The method of manufacturing a corrosion-resistant steel excellent intoughness of a base metal and a weld portion according to (3), whereinthe steel slab further contains, in % by weight, any one of, or two ormore elements selected from the group consisting of:

Cu: 0.1 to 4%;

Ni: 0.1 to 4%;

Mo: 0.01 to 1%;

V: 0.01 to 0.1%;

Nb: 0.005 to 0.050%;

Ti: 0.005 to 0.03%;

Ca: 0.0005 to 0.05%;

Mg: 0.0005 to 0.05%; and

REM: 0.001 to 0.1%.

(5) The method of manufacturing a corrosion-resistant steel excellent intoughness of a base metal and a weld portion according to (3), furtherincluding, after the step of cooling the steel plate by air, temperingthe steel plate at a temperature of A_(c1) transformation point orbelow.

(6) The method of manufacturing a corrosion-resistant steel excellent intoughness of a base metal and a weld portion according to (4), furtherincluding, after the step of cooling the steel plate by air, temperingthe steel plate at a temperature of A_(c1) transformation point orbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing relations between the Tp values (calculatedvalues of A₄ transformation point) and measured transformation points,and relations between the Tp values and presence or absence of δferrite; and

FIG. 2 is a graph showing relations between the Tc values and thetoughness (vE_(—) ₅ ) of the base metal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Paragraphs below will explain constitutive elements of the inventivecorrosion-resistant steel, concentrations thereof and the like.

C: C is an element improving the strength, but addition to an amountexceeding a predetermined level results in degradation of the toughnessin the heat affected zone (HAZ). The upper limit of the C concentrationis therefore set to 0.2%.

Si: Si is effectively added to a steel containing 2% or more of Cr, as adeoxidizer and a strengthening element, wherein the concentrationthereof less than 0.01% results in only an insufficient effect ofdeoxidization, whereas the concentration exceeding 2.0% not onlysaturates the effect but also adversely degrades the toughness of theheat affected zone (HAZ). The range of Si concentration is thereforelimited from 0.01% and 2.0%, both ends inclusive.

Cr: Cr is added in order to ensure a desirable level of corrosionresistance, similarly to Al, wherein an amount of addition of 3% or moreexhibits the effect, whereas the amount of addition exceeding 11% notonly increases the cost, but also impairs again the toughness of theheat affected zone (HAZ). The upper limit of the Cr concentration is setto 11%.

Al: Al is an important element, similar to Cr, in view of ensuring adesirable level of corrosion resistance in the present invention,wherein the concentration of Al is necessarily set to 0.1% or more inview of ensuring a desirable level of corrosion resistance. On the otherhand, the amount of addition exceeding 2% extremely widens a temperaturerange causing the ferrite phase transformation. The concentration of Alis therefore limited to 0.1% to 2%, both ends inclusive.

Mn: Mn in the present invention functions mainly as improving thestrength and as an austenite-forming element, and is added to suppressgeneration of coarse ferrite promoted by Cr and Al added in view ofimproving the corrosion resistance. More specifically, Cr and Al areferrite-forming elements as well-known, wherein large amounts ofaddition of these elements may give a ferrite single phase structureover a range from solidification point to room temperature, withoutcausing transformation, and may considerably degrade the toughness notonly in the base metal, but also in the heat affected zone (HAZ). Thepresent inventors made systematic experiments aiming at improving thetoughness of the heat affected zone (HAZ) without causing the corrosionresistance, and found out that addition of Mn can avoid the problem.Specific conditions for limitation therefor will be described later,wherein Mn is necessarily added to as much as 0.1% or more, but theamount of addition exceeding 4% enhances the hardening property, so thatthe addition is limited up to 4%.

N: The less N is contained, the more preferable, because a large amountof addition thereof to steel plate may lower the toughness of the basemetal and the heat affected zone (HAZ), so that the upper limit ofconcentration thereof is set to 0.02%.

P: The less P is contained, the more preferable, because abundancethereof lowers the toughness, so that the upper limit of concentrationthereof is set to 0.03%. The concentration thereof ascribable toinevitable contamination is preferably minimized as possible.

S: The less S is contained, the more preferable, too, because abundancethereof lowers pitting resistance, so that the upper limit ofconcentration thereof is set to 0.01%. Similarly to P, also theconcentration of S ascribable to inevitable contamination is preferablyminimized as possible.

The present invention further allows addition of the elements below.

Cu, Ni: Both of Cu and Ni exhibit effects of improving the strength, andof suppressing the ferrite generation. In particular, Ni has an effectof improving the toughness of the base metal and the heat affected zone(HAZ). Addition to as much as 0.1% or more is necessary for both of Cuand Ni in order to obtain these effects, wherein the amounts of additionof the both exceeding 4% enhances the hardenability and causesembrittlement. Both of the concentrations of Cu and Ni are therefore setto 0.1 to 4%.

Mo: Mo added to as much as 0.01% or more to a steel added with Cr and Alexhibits an effect of suppressing generation and growth of pitting,without impairing the toughness of the base metal, whereas an amount ofaddition exceeding 1.0% not only saturates the effect but also degradesthe toughness. The concentration of Mo is therefore set to 0.01% to1.0%.

Nb: Nb is an element improving the strength and toughness withoutimpairing the corrosion resistance, wherein the effect thereof isrecognizable at a concentration of as small as 0.005%, whereas theconcentration exceeding 0.05% considerably degrades the toughness of theheat affected zone (HAZ). The concentration of Nb is therefore set to0.005% to 0.05%.

V: V is an element improving the strength without impairing thecorrosion resistance, similar to Nb, wherein the effect thereof isrecognizable at a concentration of as small as 0.01% or more, whereas alarge amount of addition degrades the toughness as well-known. The upperlimit of the V concentration is set to 0.1%.

Ti: Ti is an element contributive to refinement of crystal grains athigh temperatures through production of nitride, and can particularlyimprove the toughness of the heat affected zone (HAZ), without impairingthe corrosion resistance. Both of refinement of the crystal grains andimprovement in the toughness are recognizable at a concentration of assmall as 0.005% or more, whereas addition to as much as exceeding 0.03%adversely degrades the toughness of the base metal and the heat affectedzone (HAZ), due to deposition of a large amount of carbide. The range ofconcentration is therefore set to 0.005% to 0.03%.

Ca, Mg: Ca and Mg are elements capable of improving the corrosionresistance in a steel containing Cr and Al. Although much of themechanism thereof remain unclear at present, it has been made clear thatimprovement in the corrosion resistance is recognizable at aconcentration of as small as 5 ppm or more for the both, whereas theamount of addition exceeding 500 ppm not only saturates the effect ofimproving the corrosion resistance, but also tends to degrade thetoughness. The concentrations of these elements are therefore set to 5ppm to 500 ppm, both ends inclusive.

REM: In the present invention, also appropriate addition of rare earthmetals (REM) can improve the toughness of the base metal and the weldportion, without impairing the corrosion resistance. An amount ofaddition of 0.001% or more is necessary, whereas a large amount ofaddition degrades the toughness, so that the upper limit thereof is setto 0.1%.

In the present invention, the parameter Tp expressed by the equation (1)is introduced, in order to improve the toughness of the weld portion, asone major object of the present invention.

Equation (1)

Tp=1601−(34% Cr+287% Al)+(500% C+33% Mn+60% Cu+107% Ni)

where, % Cr, % Al, % C, % Mn, % Cu and % Ni are concentrations of Cr,Al, C, Mn, Cu and Ni (% by weight), respectively.

FIG. 1 shows results of measurement and observation of transformationpoint and generation behavior of coarse ferrite, obtained when materialscomposed of a 0.015% C-0.15% Si steel (P, S and N are within the rangesof the present invention) as a base, added with Mn, Cr and Al, and forsome cases with Cu and/or Ni, were subjected to welding cycles. It isfound from FIG. 1 that generation of the coarse ferrite phase issuppressed when value of the parameter Tp, plotted on the abscissa,reaches and exceeds 1150.

The present inventors further investigated into relations betweenconcentrations of the alloying elements and the toughness, for thepurpose of ensuring a desirable level of toughness of the base metal,and found out that the toughness of the base metal can be evaluatedbased on the parameter Tc expressed by the equation (2).

Equation (2)

Tc=910+80% Al−(300% C+80% Mn+15% Cr+55% Ni)

FIG. 2 shows, together with values of the parameter Tc, results ofmeasurement of the toughness of a 0.02 to 0.05% C-0.25% Si steel as abase, added with Mn: 1.50 to 3.72%, Cr: 5.1 to 10.3% and Al: 0.8 to1.5%, wherein a 20-mm-thick steel plate was manufactured by hot rolling,and test pieces were collected from a portion having a quarter thickness(5 mm) in the longitudinal direction. It is found from FIG. 2 that arange of the parameter Tc of 600 or above can ensure an absorptionenergy at −5° C. (vE_(—) ₅ ) of as desirable as 100 J or above. Thepresent invention therefore sets the lower limit thereof to 600.

As for corrosion-resistant steel of the present invention, the steelslab can be made by the ingot making/breaking down method, continuouscasting method and the like. The steel slab may further be processed togive a steel plate by hot rolling, hot forging or the like, or may behot-worked to give an arbitrary geometry corresponding to a user's need,such as steel pipes represented by seamless steel pipe, shape steels andthe like. The hot working can be followed by air-cooling, for example.Tempering at a temperature not higher than A_(c1) transformation point,aiming at further improving the strength, will never interfere theeffects of the present invention.

The corrosion-resistant steel according to the present invention isapplicable, for example, to various corrosive environments, such ashigh-temperature and high-humidity corrosive environment, corrosiveenvironment with dewing, aerial corrosive environment, corrosiveenvironment with city water, corrosive environment with soil, corrosiveenvironment with concrete, corrosive environment with seawater, andcorrosive environments based on any combinations of them.

EXAMPLES

Each of steels having compositions listed in Table 1 was melted andcast, hot rolled to give a 15-mm-thick steel plate, wherein some of themwere further tempered, and subjected to the tests described below.

(1) Toughness Evaluation Test for Heat Affected Zone (HAZ)

All test pieces were collected from the center-thickness portion of theplate in the longitudinal direction.

Evaluation of Toughness of Base Metal: Evaluation was carried out basedon absorbed energy observed in the Charpy test at −5° C.

Evaluation of Toughness of Heat Affected Zone (HAZ): Impact test of theheat affected zone (HAZ) after being subjected to the welding heatcycles was carried out. The maximum heating temperature and the coolingrate in the test were set to 1400° C. and 15° C./s, respectively. Thebase metal was also subjected to the impact test. Transitiontemperatures were determined for the both, and ΔvTrs=([transitiontemperature of base metal]−[transition temperature after heat cycles])was determined.

(2) Corrosion Test

5-mm-thick corrosion test pieces were collected by cutting from a teststeel plate, wherein some of them were provided with Zn-base coating(coating thickness: 15 to 25 μm), and then subjected to the test underconditions described below.

Indoor Environment: The uncoated pieces were subjected to aone-hundred-day exposure test in a room with an air conditioner.

Humid Environment: The test pieces were kept at −20° C. for 2 hours, andthen kept in an environment of 95% humidity at 25° C. for 4 hours, andthis cycle was repeated 13000 times. Size of rust spot was scored forall samples.

Salt Damage Environment: The test pieces were exposed to a coastalsplash zone for 17 months.

Results of these tests are shown in Table 2. All of steels marked with Ato K are those within the scope of the present invention, and every oneof them showed a toughness of the base metal of 100 J or above, and atoughness of the heat affected zone (HAZ), evaluated in terms of ΔvTrs,of −15° C. or above, proving only a small lowering in the toughness. Asfor corrosion resistance, only a slight rusting of as small as 2 mm orless was observed on some of the pieces, and all pieces showed desirablecharacteristics.

On the contrary, all of the steels marked with L to U are thoseaccording to comparative examples out of scope of the present invention.More specifically, steels L, M and N, having the concentrations of C, Siand Mn, respectively, exceeding the upper limits specified by thepresent invention, showed almost desirable corrosion resistance, butshowed considerable degradation in the toughness. The steel marked withL showed a toughness (ΔvTrs) of the heat affected zone (HAZ) of −40° C.,indicating a large decrease. The steels marked with O and P, havingamounts of addition of Cr and Al, which are elements contributive toimprovement in the corrosion resistance, fallen below the lower limits,showed considerable decrease in the corrosion resistance. The steelmarked with Q, having the Al concentration exceeding the upper limit,showed a desirable corrosion resistance, but was degraded in thetoughness of the base metal. The steel marked with R, having Ni added asexceeding the upper limit, again showed a desirable corrosionresistance, but was poor in the toughness of the base metal. All of thesteels marked with S, T and U have the concentrations of the individualelement fallen within the ranges of the present invention, but havevalue(s) of the parameter(s) Tp and/or Tc out of the ranges of thepresent invention. More specifically, the steel marked with S is anexample having only the parameter Tp fallen out of the range of thepresent invention, showing a degraded toughness of the heat affectedzone (HAZ) of −55° C. The steel marked with T is an example having onlythe parameter Tc fallen out of the range of the present invention,showing a degraded toughness of the base metal of 83 J. The last steelmarked with U is an example having both of the parameters Tp and Tcfallen out of the ranges of the present invention, showing degradationboth in the toughness of the base metal and the heat affected zone(HAZ).

INDUSTRIAL APPLICABILITY

The present invention can provide, at low costs, a steel excellent notonly in the corrosion resistance in corrosive environment with dewing,and in other various corrosive environments such as indoor environment,aerial corrosive environment and corrosive environment with seawater,but also in the toughness of the heat affected zone (HAZ) which isimportant for weld structures, and can make a huge contribution toindustrial development.

TABLE 1 SYMBOL C Si Mn P S Cr Al N A 0.012 0.15 2.06 0.003 0.002 5.360.85 0.0045 WITHIN B 0.015 0.26 2.53 0.001 0.001 6.32 1.15 0.0077 SCOPEOF C 0.18 0.19 3.86 0.002 0.004 3.12 1.88 0.0056 THE PRESENT D 0.0120.46 2.02 0.001 0.003 10.58 0.18 0.0045 INVENTION E 0.023 0.06 0.160.006 0.001 5.93 0.72 0.0054 F 0.027 1.26 2.23 0.002 0.002 6.98 0.320.0085 G 0.038 0.28 2.88 0.002 0.001 4.89 1.13 0.0036 H 0.037 0.22 2.190.006 0.003 5.12 0.56 0.0136 I 0.019 0.87 2.38 0.003 0.002 4.44 1.020.0175 J 0.044 1.86 2.46 0.006 0.005 5.44 0.78 0.0056 K 0.025 0.55 2.040.004 0.002 4.22 1.65 0.0132 L 0.29 0.26 2.33 0.006 0.0004 6.12 0.780.0069 COMPARATIVE M 0.035 2.86 2.12 0.003 0.0007 5.66 1.23 0.0065EXAMPLE N 0.18 0.75 4.26 0.002 0.001 3.02 1.66 0.0113 O 0.018 0.26 2.460.002 0.0008 2.10 1.23 0.0056 P 0.025 0.39 2.12 0.003 0.0008 8.23 0.060.0068 Q 0.048 0.38 3.43 0.004 0.001 3.88 2.41 0.0098 R 0.023 0.23 0.580.001 0.0008 6.12 1.12 0.0075 S 0.026 0.28 3.35 0.003 0.001 6.28 1.350.0058 T 0.036 0.21 3.73 0.002 0.002 6.38 0.52 0.0068 U 0.045 0.63 3.680.006 0.003 6.88 1.71 0.0068 SYMBOL Cu Ni Mo Ti Nb V Ca Mg REM Tp Tc A1249 729 WITHIN B 0.22 1160 700 SCOPE OF C 0.59 1236 618 THE PRESENT D0.12 1262 601 INVENTION E 1.26 1344 790 F 0.042 1359 644 G 0.015 1224685 H 0.22 0.004 1381 680 I 0.86 0.96 0.028 1400 676 J 0.001 1295 681 K3.68 0.0036 1458 606 L 0.15 1391 607 COMPARATIVE M 1.82 0.001 1338 643EXAMPLE N 0.0012 1252 603 O 0.016 1267 775 P 0.0036 1386 614 Q 2.230.002 1153 633 R 0.12 4.52 1593 606 S 0.012 1124 648 T 0.012 1376 547 U0.75 0.011 1101 594 (mass %)

TABLE 2 TOUGHNESS OF CORROSION RESISTANCE WELDING- SALT HEAT-AFFECTEDINDOOR DAMAGE PORTION EN- HUMID ENVIRO- (TOUGHNE OF VIRO- ENVIRORMENTMENT BASE METAL) − MENT Zn- Zn- MANUFACTURING TOUGHNESS OF (TOUGHNESS OFNO NO BASE BASE METHOD OF BASE METAL WELD PORTION) COAT- COAT- COAT-COAT- STEEL BASE METAL vE⁻⁵ (J) ΔvTrs (° C.) ING ING ING ING WITHIN AAS-ROLLED 223 −10 ⊚ ⊚ ⊚ ◯ SCOPE OF B AS-ROLLED 253 −15 ⊚ ⊚ ⊚ ⊚ THE CAS-ROLLED 132 0 ◯ ⊚ ◯ ⊚ PRESENT D AS-ROLLED + 122 −5 ⊚ ⊚ ⊚ ⊚ INVENTIONTEMPERED E AS-ROLLED 201 −10 ⊚ ⊚ ⊚ ◯ F AS-ROLLED 248 −5 ⊚ ◯ ⊚ ⊚ GAS-ROLLED 263 −5 ⊚ ◯ ⊚ ◯ H AS-ROLLED 230 −15 ◯ ◯ ⊚ ⊚ I AS-ROLLED 213 −10⊚ ⊚ ⊚ ⊚ J AS-ROLLED + 183 −15 ◯ ◯ ⊚ ◯ TEMPERED K AS-ROLLED 198 −10 ⊚ ⊚ ⊚⊚ COMPARA- L AS-ROLLED 43 −40 ⊚ ◯ ⊚ Δ TIVE M AS-ROLLED 65 −15 ⊚ ⊚ ⊚ ⊚EXAMPLE N AS-ROLLED + 58 −15 ⊚ ◯ ◯ ⊚ TEMPERED O AS-ROLLED 216 −15 ▴ X ΔX P AS-ROLLED 263 −15 Δ X Δ X Q AS-ROLLED + 93 −20 ⊚ ⊚ ⊚ ⊚ TEMPERED RAS-ROLLED 83 −10 ⊚ ⊚ ⊚ ⊚ S AS-ROLLED 213 −55 ⊚ ⊚ ⊚ ⊚ T AS-ROLLED 83 −15⊚ ⊚ ⊚ Δ U AS-ROLLED 76 −50 ⊚ ⊚ ⊚ ⊚ ⊚: NO RUSTING ◯: RUST OF 2 mm ORSMALLER Δ: RUST OF 5 mm OR SMALLER ▴: RUST OF 10 mm OR SMALLER X: RUSTEDALMOST OVER ENTIRE SURFACE

1. A corrosion-resistant steel excellent in toughness of a base metaland a weld portion, containing, in % by weight: C: 0.2% or less; Si:0.01 to 2.0%; Mn: 0.1 to 4%; P: 0.03% or less; S: 0.01% or less; Cr: 3to 11%; Al: 0.1 to 2%; and N: 0.02% or less, and having values of 1150or above, and 600 or above respectively for Tp and Tc expressed by theequations below using concentrations of Cr, Al, C, Mn, Cu and Nirespectively given as % Cr, % Al, % C, % Mn, % Cu and % Ni.Tp=1601−(34% Cr+287% Al)+(500% C+33% Mn+60% Cu+107% Ni); andTc=910+80% Al−(300% C+80% Mn+15% Cr+55% Ni).
 2. The corrosion-resistantsteel excellent in toughness of a base metal and a weld portionaccording to claim 1, further containing, in % by weight, any one of, ortwo or more elements selected from the group consisting of: Cu: 0.1 to4%; Ni: 0.1 to 4%; Mo: 0.01 to 1%; V: 0.01 to 0.1%; Nb: 0.005 to 0.050%;Ti: 0.005 to 0.03%; Ca: 0.0005 to 0.05%; Mg: 0.0005 to 0.05%; and REM:0.001 to 0.1%.
 3. A method of manufacturing a corrosion-resistant steelexcellent in toughness of a base metal and a weld portion, comprisingthe steps of: heating a steel slab; the steel slab containing, in % byweight: C: 0.2% or less; Si: 0.01 to 2.0%; Mn: 0.1 to 4%; P: 0.03% orless; S: 0.01% or less; Cr: 3 to 11%; Al: 0.1 to 2%; and N: 0.02% orless, and having values of 1150 or above, and 600 or above respectivelyfor Tp and Tc expressed by the equations below using concentrations ofCr, Al, C, Mn, Cu and Ni respectively given as % Cr, % Al, % C, % Mn, %Cu and % Ni, forming a steel plate by hot rolling of the steel slab; andcooling the steel plate by air.Tp=1601−(34% Cr+287% Al)+(500% C+33% Mn+60% Cu+107% Ni); andTc=910+80% Al−(300% C+80% Mn+15% Cr+55% Ni).
 4. The method ofmanufacturing a corrosion-resistant steel excellent in toughness of abase metal and a weld portion according to claim 3, wherein the steelslab further contains, in % by weight, any one of, or two or moreelements selected from the group consisting of: Cu: 0.1 to 4%; Ni: 0.1to 4%; Mo: 0.01 to 1%; V: 0.01 to 0.1%; Nb: 0.005 to 0.050%; Ti: 0.005to 0.03%; Ca: 0.0005 to 0.05%; Mg: 0.0005 to 0.05%; and REM: 0.001 to0.1%.
 5. The method of manufacturing a corrosion-resistant steelexcellent in toughness of a base metal and a weld portion according toclaim 3, further comprising, after said step of cooling the steel plateby air, tempering the steel plate at a temperature of A_(c1)transformation point or below.
 6. The method of manufacturing acorrosion-resistant steel excellent in toughness of a base metal and aweld portion according to claim 4, further comprising, after said stepof cooling the steel plate by air, tempering the steel plate at atemperature of A_(c1) transformation point or below.